1) A projection in the battlements of a wall with openings through which missles can be dropped on besiegers.
(Gies, Joseph and Francis. Life in a Medieval Castle, 226)

2) Opening in floor of projecting parapet of a castle between supporting corbels, through which missles could be dropped on to assailants at base of wall. Translation of hoards into stone.
(Wood, Margaret. The English Medieval House, 413)

3) Parapet built out on corbels so that missles could be hurled through the aperture so created to prevent men attacking the base of a wall.
(Wise, Terence. Medieval Warfare, 249)